The present invention relates to a temperature responsive apparatus employing a thermocouple, and more particularly to an apparatus which has a reed switch excited by the output of the thermocouple.
It is quite well known in the art that the output voltage of a thermocouple can be easily affected significantly by a change in the circumferential or ambient temperature. Accordingly, when the thermocouple is utilized for an apparatus which is required to strictly respond to a specified temperature to be detected without any influence by the circumferential temperature, some type of temperature compensation becomes necessary.
For example, a Japanese patent publication No. 46-398l4, published on Nov. 24, 1971, discloses an apparatus providing compensation for changes in the circumferential temperature. According to this publication, particularly to FIG. 1 thereof and the accompanying description, the apparatus has a reed switch and three windings. The reed switch is connected to a voltage source through a relay winding of a relay which closes its contacts when the reed switch is closed. A first winding is connected to a thermocouple and is supplied with the thermoelectric current therefrom to generate a magnetic field through the reed switch. A second winding is connected across the voltage source and provides a magnetic field so as to bias the reed switch in the same direction as that generated by the first winding. A third winding is connected through a thermistor across the relay winding and generates a magnetic field in the opposite direction to those generated by the first and the second windings.
In this apparatus, when the output of the thermocouple reaches a predetermined value, the total magnetic field intensity generated by both the first and second windings exceeds a predetermined threshold value, so that the reed switch is closed. Current flows through the third winding in accordance with the voltage appearing across the relay. Thereby, a reverse bias is produced through the reed switch. Since the current flowing through the third winding is controlled by the thermistor, the operation of the device can be compensated in accordance with the flucutuation of the circumferential temperature.
The current, however, flows through the third winding only when the reed switch is closed; thus, there is no current for compensation of current operation until the reed switch is closed. This means that no compensation can be achieved at the time when the reed switch begins to operate.